This invention relates to a fixing device for use in an image forming apparatus such as a copying machine, a printer, and a FAX machine, and in particular, to a fixing device having a heat applying rotary body such as a heat applying roller and a heat applying fixing belt, and a pressure applying rotary body to be brought in pressing contact with said heat applying rotary body, and an image forming apparatus equipped with said fixing device.
Up to now, for a fixing device which applies heat-fixing processing to a recording material carrying toner particles composed of a resin capable of being fused by heat etc. in an image forming apparatus such as a copying machine, a printer, and a FAX machine, a heat roller method a heat belt method has been mostly employed.
A fixing device of a heat roller type is composed of a heat applying roller which is provided with a heat generation source such as a halogen heater inside and is kept in contact with a surface of a recording material carrying an unfixed toner image, and a pressure applying roller having elasticity to be brought in pressing contact with this, and by letting a recording material such as a paper sheet pass through a fixing nip portion, which is the pressing contact portion of this pair of rollers, it heats and fixes a toner image carried on the recording material.
In a fixing device which is provided in an image forming apparatus of an electrophotographic type to output a color image, a heat applying roller comprising an elastic layer of a silicone rubber or the like on the surface of its metallic roller core is used.
If the heat applying roller is of a hard nature, the surface of the heat applying roller cannot comply with the surface roughness of the toner layer and the paper sheet, and in a half-tone image, the way of toner particle crushing varies with the dot size, which produces a rough appearance of the fixed image and an unevenness of gloss in solid areas, to lower the image quality. In a monochromatic image, similar degradation of image quality occurs, but because monochromatic images have only black color and are mostly centered in a line image or non-glossy image, the above-mentioned defects are comparatively remarkable. For this reason, in order to make color image quality high, a fixing heat roller having an elastic layer is essential.
In a color image forming apparatus equipped with a fixing device of a roller fixing type composed of a heat applying roller having thermally conductive base member (metallic core) covered with an elastic layer thereon and further a releasing layer on it, and a pressure applying roller, for the releasing layer lying on the surface, a fluorine-contained resin, a fluorine-contained rubber, a silicone rubber, etc. are employed. In the case of an image forming apparatus in which images are formed only on one side of paper sheets, it is necessary to select the releasing layer on the surface of the heat applying roller with the following factors taken into consideration; those are image quality, toner offset characteristic, the tendency of a sheet paper to coil round the heat applying roller, durability, etc.
On the other hand, in respect of the releasing layer lying on the surface of the pressure applying roller, only it has been necessary to select its material with attention paid only to durability, toner offset characteristic for preventing toner particles, which have been once offset to the heat applying roller, from transferring to the heat applying roller, and the inclination of a paper sheet to wind itself round the pressure applying roller. In recent years, in a color image forming apparatus capable of duplex copying which has been in process of development, it is necessary to pay attention to the quality of first-side images, and concerning also the releasing layer on the surface of the pressure applying roller, it has now become necessary to select it with attention paid to image quality etc. At present, it is general to make the condition of the releasing layer of the pressure applying roller the same as the releasing layer of the heat applying roller. However, this condition is of no problem as long as unfixed toner images which have been formed on both sides of a transfer material is fixed at a time, but in the case where a toner image which has been formed on the second side of a transfer material is fixed by a fixing device again, after a toner image which has been formed on the first side of the transfer material is once fixed by the fixing device, the first side of the transfer material is to be subjected to fixing process two times, which produces degradation of image quality. Regarding the hardness of the releasing layer, it is found that micro-hardness is specified in the publication of the unexamined patent application 2000-75714; however in this specification, micro-hardness of the surface of a heat applying roller is specified for the purpose of forming a simplex image on an OHP sheet, and it is not related to the solution of the problems in forming duplex copy images as this invention.
Further, a heat applying roller, which is described in the publication of the examined patent application H6-100876 is formed of four layers, which are a roller base member, a silicone rubber layer, a mixture layer composed of a rubber and a resin, and a resin layer, and by a mixture layer as an intermediate layer, the silicone rubber layer lying below and the resin layer lying above are firmly bonded. Because there is neither specification about the thickness and hardness of this heating roller formed of four layers, nor specification about the structure of the pressure applying roller, it does not help to solve the problem in such a case of duplex copy formation as this invention.
Further, in the publications of the unexamined patent application S61-22376, the relation of the thickness values between the releasing layers of the heat applying roller and the pressure applying roller is specified, but the rollers have a structure such that the thickness of the surface releasing layer of the heat applying roller is made smaller than that of the surface releasing layer of the pressure applying roller, which makes worse the compliance of the pressure applying roller surface to the surface roughness of the paper sheet used and the toner layer than that of the heat applying roller; therefore, it does not help to solve the problem in a case of duplex copy formation such as this invention.
It is an object of this invention to provide a fixing device and an image forming apparatus solving the problems which have been there up to now as described in the above.
The above-mentioned object is accomplished by any one of the structures described below.
Structure 1: A fixing device for use in an image forming apparatus capable of making a duplex copy by a paper sheet inverting conveyance means comprising a heat applying rotary body having at least one elastic rubber layer or more and a toner releasing layer on the surface for heating by contact the one side of a transfer material carrying an unfixed toner image, a pressure applying roller having at least one elastic rubber layer or more and a toner releasing layer on the surface for conveying a transfer material for fixing as kept in rotary contact with the heat applying rotary body, a drive source for driving at least one of said rotary bodies, and a heat applying source for heating at least one of said rotary bodies, characterized in that the micro-hardness of the surface of said pressure applying rotary body is made smaller than the micro-hardness of the surface of said heat applying rotary body.
Structure 2: A fixing device as set forth in structure 1 characterized in that the micro-hardness of the surface of the aforesaid pressure applying rotary body in the state where the both of the aforesaid heat applying rotary body and said pressure applying rotary body are not provided with a surface layer is not smaller than the micro-hardness of the surface of said heat applying rotary body.
Structure 3: A fixing device as set forth in structure 1, characterized in that the value of the aforesaid micro-hardness is a value in the case where the elastic rubber layers of the aforesaid heat applying rotary body and the aforesaid pressure applying rotary body are formed of the same material to have the same thickness.
Structure 4: A fixing device as set forth in any one of structures 1 to 3, characterized in that the releasing layer of the aforesaid heat applying rotary body and that of the aforesaid pressure applying rotary body are formed of the same material, and the thickness of the releasing layer of said pressure applying rotary body is made smaller than the thickness of the releasing layer of said heat applying rotary body.
Structure 5: A fixing device as set forth in any one of structures 1 to 3, characterized in that the hardness of the releasing layer of the aforesaid pressure applying rotary body is made smaller than the hardness of the releasing layer of the aforesaid heat applying rotary body.
Structure 6: A fixing device as set forth in any one of structures 1 to 3, characterized in that the aforesaid heat applying rotary body and the aforesaid pressure applying rotary body have at least one or more mixture layers composed of rubber and resin between the elastic rubber layer and the releasing layer, and in the case where the releasing layers are formed of the same material to have the same thickness, the thickness of the mixture layer of said pressure applying rotary body is made smaller than the thickness of the mixture layer of said heat applying rotary body.
Structure 7: A fixing device as set forth in any one of structures 1 to 3, characterized in that the aforesaid heat applying rotary body and the aforesaid pressure applying rotary body have at least one or more mixture layers composed of rubber and resin between the elastic rubber layer and the releasing layer, and in the case where the releasing layers are formed of the same material to have the same thickness, the hardness of the mixture layer of said pressure applying rotary body is made smaller than the hardness of the mixture layer of said heat applying rotary body.
Structure 8: A fixing device as set forth in any one of structures 1 to 7, characterized in that both of the aforesaid heat applying rotary body and the aforesaid pressure applying rotary body have a shape of a roll having a stiff base member inside the elastic layer.
Structure 9: A fixing device as set forth in structure 8, characterized in that the Asker-C hardness of the aforesaid pressure applying rotary body is made larger than the Asker-C hardness of the aforesaid heat applying rotary body.
Structure 10: A fixing device as set forth in structure 8, characterized in that the elastic layer of the aforesaid pressure applying rotary body and the elastic layer of the aforesaid heat applying rotary body are formed to have the same Asker-C hardness per unit thickness, and the thickness of the elastic layer of said heat applying rotary body is made larger than the thickness of the elastic layer of said pressure applying rotary body.
Structure 11: A fixing device as set forth in structure 8, characterized in that the elastic layer of the aforesaid pressure applying rotary body and the elastic layer of the aforesaid heat applying rotary body are formed to have the same thickness, and Asker-C hardness of the elastic layer of said heat applying rotary body is made smaller than the Asker-C hardness of the elastic layer of said pressure applying rotary body.
Structure 12: A fixing device as set forth in any one of structures 1 to 7, characterized in that the aforesaid heat applying rotary body is a heat applying fixing belt having a shape of an endless belt, and the two aforesaid pressure applying rotary bodies both have a shape of a roll comprising a stiff base member inside the elastic layer.
Structure 13: A fixing device as set forth in structure 12, characterized in that said fixing device further comprises a heat applying fixing belt pressing body kept in contact with the inner side of the aforesaid heat applying fixing body for pressing the heat applying fixing belt to the aforesaid pressure applying rotary body.
Structure 14: A fixing device as set forth in structure 13, characterized in that the Asker-C hardness of the aforesaid pressure applying rotary body is made larger than the Asker-C hardness of the aforesaid heat applying fixing belt pressing body.
Structure 15: An image forming apparatus equipped with a fixing device as set forth in any one of structures 1 to 14, characterized in that said image forming apparatus comprises an image forming means capable of outputting an image having at least two or more colors.
Structure 16: An image forming apparatus as set forth in any one of structures 1 to 15, characterized in that the surface layer of the aforesaid heat applying rotary body and the aforesaid pressure applying rotary body is mainly composed of PFA, the toner for use in said apparatus includes a wax, and the fixing device has no mechanism for coating the surface of the heat applying rotary body and the surface of the pressure applying rotary body with a releasing agent.
Further, preferable structures are as follows:
(1) A fixing device comprising a heat applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon, a pressure applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon to be brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the releasing layer of said heat applying rotary body and the releasing layer of said pressure applying rotary body are formed of the same material, and the thickness of the releasing layer of said pressure applying rotary body is smaller than the thickness of the releasing layer of said heat applying rotary body.
(2) A fixing device comprising a heat applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon, a pressure applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon to be brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the micro-hardness of said pressure applying rotary body is made smaller than the micro-hardness of said heat applying rotary body.
(3) A fixing device comprising a heat applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon, a pressure applying rotary body having a thermally conductive base member coated with an elastic layer and further a releasing layer thereon to be brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the hardness of the releasing layer of said pressure applying rotary body is made smaller than the hardness of the releasing layer of said heat applying rotary body.
(4) A fixing device comprising a heat applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin thereon, and further a resin layer thereon, and is heated by a heat applying source, a pressure applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin, and further a resin layer thereon, and is brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the resin layer of said heat applying rotary body and the resin layer of said pressure applying rotary body are formed of the same material, the resin layer of said heat applying rotary body and the resin layer of said pressure applying rotary body are formed to have the same thickness, and the thickness of the mixture layer of said pressure applying rotary body is made smaller than the thickness of the mixture layer of said heat applying rotary body.
(5) A fixing device comprising a heat applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin thereon, and further a resin layer thereon, and is heated by a heat applying source, a pressure applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin, and further a resin layer thereon, and is brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the micro-hardness of said pressure applying rotary body is made smaller than the micro-hardness of said heat applying rotary body.
(6) A fixing device comprising a heat applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin thereon, and further a resin layer thereon, and is heated by a heat applying source, a pressure applying rotary body, which has a thermally conductive base member coated with an elastic layer, further a mixture layer composed of a rubber and a resin, and further a resin layer thereon, and is brought in pressing contact with said heat applying rotary body, and a drive source for driving at least one of said rotary bodies, characterized in that the resin layer of said heat applying rotary body and the resin layer of said pressure applying rotary body are formed of the same material, the resin layer of said heat applying rotary body and the resin layer of said pressure applying rotary body are formed to have the same thickness, and the hardness of the mixture layer of said pressure applying rotary body is made smaller than the hardness of the mixture layer of said heat applying rotary body.
(7) An image forming apparatus characterized in that it is equipped with a fixing device as set forth in any one of the above-mentioned structures (1) to (6), means for forming an image, and means for conveying a paper sheet.